Pre-filter cotton is graded G2, G3 or G4 under the old EN 779 scheme (roughly ISO Coarse 30–60% today), and buyers regularly pay for a grade they don’t need — or choke a system with one grade too fine. Here is what the letters actually buy you.
What the Grades Mean in Practice
G2 catches lint, insects and visible dust — think of it as a wind sock for your intake. G3 is the general-purpose workhorse: leaves, sanding dust, most of what a workshop throws at an intake. G4 adds real capture of finer dust and is the standard first stage ahead of F-class filters in booths and air handlers. Each step up costs a little more pressure drop from day one and clogs a little sooner in dirty air.
Match the Grade to the Job
- Spray booth intake: G3 or G4, always paired with the F5 ceiling — the pre-filter’s whole purpose is to keep the expensive ceiling media alive.
- AHU / HVAC first stage: G3 roll media cut to the frame; step to G4 where coils have historically fouled.
- Compressor and machine rooms: G2–G3, changed often, beats G4 changed rarely.
- Dusty rural or construction-adjacent sites: two coarse stages (G2 then G4) outlast one fine stage every time.
Thickness and Density Matter as Much as Grade
Two G4 medias can behave very differently: a thick, lofty G4 holds far more dust between changes than a thin dense one at the same rating. That dust-holding capacity — not the grade letter — is what sets your changeout interval. It is also where cheap media cheats: hitting the grade on day one with half the loft, then blinding over in weeks.
A Simple Change Rule
If you have gauges, change at roughly double the clean pressure drop. If you don’t, hold the media up to a work light: when the light stops passing through the dirty face, so does your air. Keep a season of spares on the shelf — pre-filter cotton is the cheapest component in the system and the most expensive one to run out of.
Unsure between G3 and G4 for your line? Send a photo of the intake and the environment to WhatsApp +86 186 1558 6932 and we will recommend a grade and thickness — with samples to prove it.